The invention relates to the fluxless joining of flip chip devices on laminates with plated solder. The invention also relates to the protection of such solder joinable surfaces against excess oxidation.
Fluxes are commonly reacted with surface oxides, such as tin oxide, during the reflow process to generate metal surfaces which subsequently intermix forming a solder joint. Problems in fluxing and soldering can arise in which inadequate amounts of flux or excessive thicknesses of tin oxide are present either or both of which can inhibit wetting and intermixing and result in the formation of non wetted, electrically open solder joints. In addition, where excess flux may be incompletely consumed in the reflow process. The remaining residues will interfere with the efficiency of subsequent process steps such as chip underfill.
Routinely, tin oxide is controlled by multiple reflows of solder in the presence of flux which subsequently must be removed, either by aqueous, organic solvent, or gaseous cleaning. The newly-cleaned surfaces are now subject to reoxidation by exposure to ambient air, thus the thickness of tin oxide layer may vary unacceptably in an uncontrolled manner.
Currently, a variety of flux types, both those that do not require a separate cleaning step, no clean, and types that are removed by aqueous, or organic solvent washing are employed in the electronics industry. These fluxes introduce residues onto laminate, solder joint and chip surfaces which can inhibit proper underfill flow required for subsequent mechanical stability of the solder joints.
This invention selectively precoats the solder surfaces of either the chip, the laminate, or both with a metal-complexing agent in a flux which reacts with the tin oxide in order to control the reactivity of tin at the solder surface. This complexing agent forms a continuous thin metal carboxylate film on the solder surfaces and protects the solder surface from further oxide formation. In this form the chips and laminates may be handled in a routine manner without particular concern for oxide thickness growth as long as mechanical damage to the solder surfaces is avoided. When ready for joining the chip solder bumps and laminate solder pads are brought into intimate contact and subjected to reflow temperatures. The tin carboxylate complex facilitates solder reflow, but moreover, when exposed to peak reflow temperatures, decomposes into volatile products, leaving a clean surface easily wetted by the solder.
This invention may be carried out applying additional flux just prior to reflow, or the additional flux application may be omitted resulting in a fluxless joining process. Where the flux is omitted, the treated chips or laminates, now having protected surfaces, may be safely stored for later final assembly. Advantages of the invention include control of the tin oxide formation prior to chip joining, handling improvement, opportunity for fluxless joining and elimination of residues after reflow promoting underfill adhesion.
The invention provides a method of protecting tin solderable surfaces. To a solderable surface, coated with tin oxide, a complexing agent is applied. The complexing agent is, a dicarboxylic acid, for example, pimelic acid. Heating drives the formation of tin pimelate which forms a tenacious film over the solder surface protecting it from reacting further with ambient oxygen. The protective agent, tin pimelate in this example, also prevents the oxidation of Sn(II) to Sn(IV). Sn(IV) is well known to be difficult to complex and to inhibit the solder process.
The invention provides a means of generating clean metal surfaces for solder joining without incurring problems associated with the use of customary flux. An aspect of the invention is that the temperatures associated with reflow decompose the protective agent to volatile materials, in the present example, tin pimelate is decomposed to cyclohexanone and CO2. The decomposition results in a clean surface.
An additional aspect of the invention is provision of means of protection against oxidation of solderable surfaces during storage awaiting final assembly.
Yet a further aspect of the invention is provision of means by which flux contamination of extraneous surfaces is prevented. The present invention eliminates the need for flux, usually required for multiple reflow steps, and minimize the further growth of the tin oxide layer. The present invention substantially eliminates flux residues since the decomposition products are gaseous.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.